This application relates generally to systems and methods for combining two or more signals and particularly to systems and methods for converting multiple baseband sampled data streams into a frequency division multiplexed intermediate frequency (IF) signal ready to be converted to analog form in a typical RF communication system.
It is often desirable in communication systems to transmit a composite signal which is a combination of two or more data signals, combined in a predetermined manner. The data signals may represent voice communications, binary data communications, facsimile data, modem telecommunications, and similar baseband signals. For example, it is well known in the at to use time division multiplexing or frequency division multiplexing to generate a composite signal in which individual data signals are allocated a predetermined port,on of the time or frequency spectrum within the composite signal. A particular example of such a system is the transmit portion of a cellular base station. As is well known, the base station communicates both voice and data signals with plural remote stations simultaneously. To avoid interference between the various signals being transmitted to the mobile stations, the cellular base station communicates with each of the mobile stations using a separate RF carrier frequency. Typically, present base stations handle 100 to 300 mobile stations simulaneously.
One method to achieve such simultaneous transmission is to use separate transmitters for each signal to be transmitted, each transmitter being tuned to a predetermined frequency. Obviously, the use of a couple of hundred transmitters presents both space and cost problems. Accordingly, typical present cellular systems frequency multiplex the individual signals destined for the remote stations and broadcast only the composite signal. Each mobile station is then free to receive the composite signal and demodulate it to decode and use the signal present at the frequency then assigned to the particular mobile unit by the conventional cellular control system.
To a certain extent, conventional cellular base stations process the signals to be transmitted in parallel, each signal being modulated to a separate intermediate frequency ("IF"), each intermediate frequency being spaced in frequency from adjacent frequencies by the final desired frequency separation. After the individual signals are modulated up to the intermediate frequencies, the individual signals are combined into a composite IF signal which is then up converted (or modulated) to the desired radio frequency ("RF")and transmitted.
By way of an example, in one present system, each cellular base station transmits 124 signals (or "channels") simultaneously. Each channel is 180 KHz wide on center frequencies separated by 200 KHz. The RF band used for transmission of the composite signal is from 935 MHz to 960 MHz. In such a system, it is possible to use a 20 MHz composite IF center frequency. Under this condition, the first (lowest frequency) baseband channel would be up converted to a 7.7 MHz center IF frequency. The second baseband channel would be up converted to a 7.9 MHz center IF frequency. The last baseband channel would be up converted to a 32.3 MHz center IF frequency. The individual IF channels are then summed and the composite signal is up converted to a 947.5 MHz center frequency for transmission by convention RF transmission equipment.
In known cellular communications systems, the baseband data is generally in digital format, often but not necessarily the result of a digital sampling of a voice or data communication. Some prior art cellular base stations convert the individual digital data signals to an analog format. Often conversion is done in a digital baseband processor which modulates each digital modulating signal by the digital data signal and supplies each of the modulated signals to individual digital-to-analog converters ("DAC") and filtered. Each of the individual analog signals is then up converted to its assigned IF frequency and subsequently summed with the other 123 channels (which have undergone the same process in parallel). The composite signal may then be modulated to the desired RF frequency for subsequent transmission. Note that the prior art analog method requires 124 separate DACs, smoothing filters, and up converters. In addition, the prior art analog method requires a very complicated 124 input analog summing tree.
It is known in the prior art analog methods to reduce the number of analog signal required by combining multiple digital modulated channels in the baseband processor. However, even with such a reduction, prior art analog systems are often considered difficult and costly to implement particularly in cellular base stations in which component and system performance is tightly specified. To achieve the tight specifications, such prior art systems have to be rigorously designed and manufactured to reduce the effect of spurious signals generated by each of the DACs and, in general, by each analog step in the system.
Accordingly, it is known to use digital signal processing techniques to circumvent many of the problems associated with analog signal processing systems. In a digital signal processing system, the individual data signals, each digitally modulated to a separate IF frequency, are summed digitally and the composite signal thereby digitally generated. The composite digital signal may then be converted to an analog signal, modulated to an RF signal, amplified and transmitted. Typically, in such digital systems, each of the individual modulated signals is digitally sampled at the same sampling rate and at a rate which is consistent with the Nyquist sampling rate for the expected composite signal. So long as the sampling rate criteria have been met, the individual signals may be passed to a digital summing tree for combining into the composite signal. If a digital equivalent to the analog implementation is used, a system structure such as shown in summing tree of FIG. 1 may result.
With reference to a typical digital summing tree as shown in FIG. 1, each channel has a predetermined precision, 12 bits in the system of FIG. 1. As the channels are added together by the adder, additional bits must be added to the circuit for each adder to retain the precision of the sum. For a 124 channel signal, a summing tree will generally require 7 layers, resulting in a 19 bit precision from the final adder in the tree. If desired, the composite signal from the final adder may by rounded to its most significant bits prior to converting to an analog signal. In practice, the circuit of FIG. 1 with the width of its adders growing for each level requires a relatively large amount of hardware to implement. In addition, some of the precision of the prior art circuit is lost if not all the channels are active and the rounder circuit automatically lops off the least significant bits of the composite signal.
It is therefore an object of the present invention to provide a novel system and method for combining digital signals with a reduced complexity in comparison to prior art systems.
It is yet another object of the present invention to provide a novel system and method in which multiple communication signals may be combined without a loss of precision.
It is still a further object of the present invention to provide a novel system and method in which multiple signals may be combined by repeated use of similar components.
These and many other objects and advantages of the present invention will be readily apparent to one skilled in the art to which the invention pertains from a perusal of the claims, the appended drawings, and the following detailed description of preferred embodiments.